Unlike other ancient religions, Sanatan Dharma has never vanished completely from Bharat; it got eclipsed though and with its eclipsing India got weakened.
The reason for this eclipse was mainly a fatigue that set up following a long period of creative impulsion. The sign of this fatigue was the world shunning illusionism becoming a mainstream Vedantic thought and the rejection of Shakti from life leading eventually to the diminution of spiritual force and energy into small and narrow other-worldly streams. This too was needed for the world to go through a period of darkness during which the Asuric forces invaded India. All these invasions and intermingling challenged the Indian thought and way of life with all that contradicts it. Yet even during the peak of these invasions following the post-Buddhist decline of the intense vitality of Indian culture, the Sanatan Dharma never died. It simply withdrew into caves and sects and cults as if secretly preparing to bounce back. And when it began to bounce back against the invading forces it was primarily on the basis of its spiritual strength as we see in the life of Shivaji and Guru Govind Singh who stood against the Mughals and Sri Aurobindo and a number of other spiritually oriented revolutionaries who stood against the British Raj. 
The Mughalsand the British were defeated by the awakening spiritual strength of India and as we can see now that as the Renaissance and uprise of India is underway there is also an increasing interest in the Sanatan Dharma, not only in India but all over the world. What was destroyed were the old forms but the spiritual aspiration that gave birth to the Sanatan Dharma, its core was never lost. It can never be lost. Even with pralaya it is only the old forms that are lost and never the core truth much like as in death, it is the body that dies but the soul returns back stronger and wiser into a new body. This goes on until the soul recovers its full glory and divine strength and can fill the body as well with its immortal substance. When it can do that then pralaya will no more be needed nor the decline from Satyuga to Kaliyuga.
Sri Aurobindo and the Mother assure us based on their spiritual realisation and tapasya that the time has now come when pralayas and the law of decline and death before taking a new form will no more be needed and we, at least a sizeable number of human beings will be able to grow from Light to greater Light and under their pressure earth and the human body itself will eventually escape the law of decline and death. What has gone before us were ages of preparation. What is going on now is the Age of spiritual ascension. What is going to come are the Ages of divine fulfilment.
